Cache2 failed to add to the pool, this sometimes happens for whatever reason, can't say why without the diags from when it was first added, but it's usually fixed by adding it again, stop array, unassign cache2, start array, stop array, re-assign cache2, start array, look for activity on cache2 or just post new diags.
Note: this is considered a safe procedure but still make sure cache backups are up to date before doing it.